Our objective in this project is to achieve consistent engraftment of allografts from haploidentical donors with minimal regimen-related toxicity. It is based on the hypothesis there exists a level of host T cell depletion, which can be quantified and obtained without myeloablative conditioning, sufficient to permit consistent engraftment of haploidentical allografts. To achieve our objective and test this hypothesis, we developed a novel treatment approach, referred to as "targeted immune-depletion", which consists of two major components: a pre-transplant induction regimen and a reduced-intensity transplant conditioning regimen. The induction regimen consists of disease-specific immunosuppressive chemotherapy at conventional doses that is administered until host T cell depletion to a specific pre-determined level has been achieved and precedes a reduced-intensity conditioning regimen and alloHSCT. Targeted immune-depletion offers the following advantages: 1) host T cell depletion in a patient-specific manner, 2) disease control, and 3) minimal regimen-related toxicity. This strategy has been implemented through a systematic progression of clinical trials, in which individual factors that affect engraftment have been modified in a step-wise fashion. We first tested targeted immune-depletion in the setting of T cell replete allografts from HLA-matched siblings in protocol 99-C-0143. The induction regimen, EPOCH-F, resulted in disease control and significant host T cell depletion, and rapid and complete donor chimerism was observed after reduced-intensity alloHSCT. Next, in protocol 00-C-0119, we tested the approach with ex-vivo T cell depleted (TCD) allografts from HLA-matched siblings. This strategy resulted in sustained donor engraftment in all patients. In accordance with our hypothesis, mixed chimerism correlated with pre-transplant circulating T cell numbers. Based upon these results we have initiated a study, 04-C-0116, of targeted immune-depletion prior to reduced-intensity alloHSCT using TCD allografts from HLA-mismatched related donors. Concurrently, as part of protocol 04-C-0131, we are investigating Th2/Tc2 cells with TCD alloHSCT from HLA-matched sibling donors. Th2/Tc2 cells abrogate rejection with reduced GVHD, and our plan is to eventually incorporate this cellular product into our studies in haploidentical HSCT.